A major application of nanomedicine lies in the early detection of biomolecules (biological molecules) that are molecular markers of malignant diseases. Methods currently available for detection of disease biomarkers include fluorescence immunoassays, enzyme-linked immunosorbent assays (ELISA), PCR approaches, biobarcode assays, quartz crystal microbalance analyses, electrochemical methods, microcantilever detection, and nanoparticle-based biosensors. Despite the diversity of available detection platforms, important challenges still remain in minimizing sensor size, reducing detection time, eliminating target labeling requirements, minimizing signal amplification steps, and developing simple and inexpensive fabrication protocols.
Quantitative characterization of biomolecules, such as metabolic intermediates, proteins, lipids, and nucleic acids, is a critical aspect of molecular diagnostics and drug development (Urdea et al., 2006, Nature 444: 73-79; Frost et al., 2002, Curr. Opin. Chem. Biol. 6: 633-641). A number of assays based on spectrophotometry, fluorometry, chemiluminiscence, and electrochemical immunoassays have been reported for biomolecular detection (Capitan-Vallvey, 2001, Anal. Chim. Acta 433: 155-163; Guo et al., 2000, Anal. Chim. Acta 403: 225-233; Chen et al., 2004, Anal. Chim. Acta 521: 9-15; Tsukagoshi et al., 2004, Anal. Chem. 76: 4410-4415; de la Escosura-Muniz et al., 2004, Anal. Chim. Acta 524: 355-363; Palecek et al., 2004, Anal. Chem. 76: 5930-5936; Diaz-Gonzalez et al., 2005, Talanta 65: 565-573). These methods are effective, but are often slow due to multiple sample pretreatment steps that may increase labor, time, and cost of the analysis. Immunoassays that combine high sensitivity with fast, robust, and inexpensive methodologies for biomolecular detection and analysis are of growing importance (Kricka, 1998, Clin. Chem. 44: 2008-2014). One such example is the biosensor for the detection of S-adenosyl homocysteine (SAH), a diagnostic marker for cardiovascular disease (Melnyk et al., 2000, J. Biol. Chem. 275: 29318-29323).
Increased attention has recently been paid to the development and application of separation techniques that employ small magnetic beads to separate a desired analyte from a complex biological sample. Magnetic beads have been employed for DNA isolation (followed by PCR amplification), genomic analyses drug discovery, and affinity capture and purification of proteins and peptides. Also, magnetic beads coupled to Ni-NTA (nitrilotriacetic acid) ligands have been used in the magnetocapture, purification, and detection of histidine-tagged proteins. Although magnetic bead-based strategies have greatly improved analyte capture, their potential for biosensing applications is limited by the labor and time required to quantitate the captured biomarker. To this end, combining a one-step magnetic bead-based capture scheme with a fluorescence or radiolabel-free detection scheme would constitute a significant advance.
Biomolecular detection using optical diffraction gratings is a versatile label-free method (Brandenburg et al., 2000, Appl. Optics 39: 6396-6405). However, the microfabrication of diffraction gratings is a tedious and expensive step that limits its practical applicability (Bailey et al. 2003, J. Am. Chem. Soc. 125: 13541-13547). Also, most of the available diffraction grating-based detection methods require additional signal amplification steps (after the desired analyte is captured on a solid surface), such as the sequential use of nanoparticle-conjugated oligonucleotides or antibodies and/or the use of enzymes to improve the sensitivity of detection, leading to time-consuming, multi-step procedures (Loo et al., 2005, Anal. Biochem. 337: 338-342).